1. Field of the Invention
This invention relates to a steel cord of novel twist construction for use in reinforcing automobile tires, conveyor belts, etc., as well as a tire reinforced by such steel cords.
2. Description of the Prior Art
Conventionally, steel cords composed of four or five material wires intertwisted, namely, of the construction of 1.times.4 or 1.times.5, have been used for a reinforcing layer of a belt part of a steel radial tire. However, as shown in FIG. 18, since a steel cord (12) of 1.times.5 construction, for example, is composed of five material wires (11) twisted closely together, when a tire is formed rubber does not penetrate into a cavity (X) at the central part of a steel cord and thus the cavity (X) is left void along the lengthwise direction of the steel cord. Accordingly, in the case of tires using such steel cords, when the tire is damaged externally during the running of an automobile, water penetrates into the cavity (X) of the steel cord through cracks and further penetrates along the lengthwise direction of the steel cord, causing rusting of the steel cord and lowering of adhesiveness between the steel cord and rubber. Thus, the so-called "separation phenomenon" (in which the steel cord and rubber are separated from each other) occurs and the usable life of the tire is significantly shortened. In order to prolong the usable life of automobile tires and to make tires lighter in weight, it has been required recently to improve corrosion resistance and break-resistance of steel cords. Also, the so-called "open cord" with improved adhesiveness between steel cord and rubber by making rubber penetrate deeply into the steel cord has been developed.
The above-mentioned steel cord, as shown in FIG. 19, is substantially circular in cross section and is composed of five material wires (11) intertwisted in such a fashion that a clearance (Y) exists between material wires. However, since this steel cord is constructed with a substantially circular cross section and has a large amount of free space which allows movement of material wires, in forming tires such free space is reduced due to tension applied to the steel cord, with the result that rubber cannot penetrate fully into the steel cord and adhesiveness between material wires and rubber is lowered. Moreover, during of vulcanizing of the rubber, material wires of the above-mentioned steel cord are elongated and are turned due to tensile forces caused by flowing of the rubber as it is melted or softened by heating or due to a twisting force in the lengthwise direction of the steel cord, such that clearance between the material wires are reduced. Thus, rubber cannot penetrate fully into the steel cord, causing corrosion and "separation phenomenon" to occur in the steel cord such that the usable life of the tire is shortened.
When a tire runs on pebbles, carbstone or the like, steel cords are liable to break. In order to improve break-resistance of steel cords, it has been suggested to use a so-called "high elongation cord" (steel cord which allows a belt part of the tire to undergo significant elongation). At the present time, steel cords having plural twisted layers (twist construction of 3.times.7, 4.times.2, 4.times.4, etc.) and a relatively small twist pitch are used as high elongation cords for belt parts of tires for trucks and buses. Also, steel cords of a single twisted layer (1 .times.4, 1.times.5, etc.) are used.
However, the above steel cords having multiple or plural twisted layers are faced with disadvantages such as low productivity and high manufacturing cost due to the small twist pitch and the plural twisting. In the case of steel cords having a single twisted layer, they have disadvantages such as low elongation before breakage (e.g. 2.5-3.8%) such that they cannot be used as high elongation cords.